The present invention relates to a reference voltage measuring bridge for a device for monitoring and measuring the insulation of a DC voltage electrical mains system, notably a low voltage system, for example about 600 volts, the voltage measuring bridge being designed to supply at least a portion of the useful input signal of an analysis channel of a reference AC voltage, of low frequency, comprised between 4 and 10 Hertz for example, and of low voltage, about 30 volts peak-to-peak for example, which is input to the DC electrical mains system.
It is state-of-the-art, for example from the documents FR-A-2,616,228 and FR-A-2,647,220, to use for monitoring and measuring the insulation of an electrical mains system with respect to earth, a device comprising an input circuit of a reference AC signal to the mains system to be monitored, and an electronic detection circuit able to supply a measurement signal representative of the insulation state of the mains system in response to sending of the reference signal, this insulation state being illustrated by a leakage impedance Zd formed by a leakage resistance Rf electrically connected in parallel to a stray capacitor Cf, whose capacity depends on the mains system characteristics.
The accompanying FIG. 1 represents the input circuit of the state-of-the-art insulation monitor in schematic form.
This involves in FIG. 1 a mains system supply DC voltage Vc of rather high amplitude, for example 600 volts. The voltage is applied between a line wire 1 and a return wire 2 which is insulated from the installation ground. The insulation impedance Zd between the two DC power supply wires 1 and 2 is composed of the leakage resistance Rf and the stray capacitor Cf, whose capacity depends on the mains system characteristics and which, as represented, is connected in parallel to the resistance Rf.
The insulation monitor, whose object is to monitor the two components Rf and Cf of the insulation impedance Zd, comprises means (not shown) for injecting a reference AC voltage Vd, via an input wire 4 and a coupling capacitor 3, between the line wire 1 and the installation ground. This AC voltage is of low frequency f, generally comprised between 4 and 10 Hertz, and has a relatively low peak-to-peak voltage, about thirty volts for example.
Determining the resistive value Rf and the capacitive value Cf of the impedance Zd requires both the value of the input voltage Vd and that of the corresponding current Id flowing in the insulation impedance Zd to be measured.
The current Id is measured by taking the value of the voltage at frequency f which is due to the current Id flowing in a measuring resistance R3 serially connected with the impedance Zd, between the return wire 2 and ground. The information is applied to an input El of the processing and display unit 5 which equips the insulation monitor.
The voltage Vd is measured, and applied to another input E2 of the unit 5, by taking it at the potentiometric output connector 6 of a dividing bridge with two resistances R1, R2, whose attenuation ratio, which is about 80 for example, and total resistance (R1=R2), which is about 600 kiloohms for example, are chosen in such a way as to achieve a compromise between:
the measuring precision which is greater the higher the ratio R2/R1 (high AC voltage input on the input E2) and the lower the resistance (R1+R2) of the bridge 7 (high alternating current flowing at this point); PA1 the input impedance of the DC mains system which must not be reduced too much by the presence of this bridge 7; PA1 the DC voltage level at the measuring point 6, which must not only be lower than the operating voltage of the semi-conducting components of the unit 5 (+15 volts, -15 volts in general), but which must also not be high with respect to that of the input AC voltage measured at the same point 6.
The solution described in the document FR-A-2,647,220 which consists in connecting a capacitor between the resistance R1 and the line conductor 1 gives rise to a stray phase displacement problem, liable to disturb the measuring precision.
Adopting a compromise of this kind in choosing the values of the resistances R1 and R2 cannot be totally satisfactory. The resulting impedance of the bridge 7 is in fact considered to be insufficient, and the useful AC signal taken at the mid-point 6 is so weak that it is practically drowned in the noise. The residual DC power supply at the mid-point 6 is much too high, both in voltage and current intensity values.